US5660917A - Thermal conductivity sheet - Google Patents

Thermal conductivity sheet Download PDF

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Publication number
US5660917A
US5660917A US08/393,007 US39300795A US5660917A US 5660917 A US5660917 A US 5660917A US 39300795 A US39300795 A US 39300795A US 5660917 A US5660917 A US 5660917A
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United States
Prior art keywords
thermal conductivity
thermally conductive
sheet
conductivity sheet
insulators
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US08/393,007
Inventor
Yoshinori Fujimori
Jun Momma
Tomiya Sasaki
Hideo Iwasaki
Toshiya Sakamoto
Hiroshi Endo
Katsumi Hisano
Naoyuki Sori
Kazumi Shimotori
Noriaki Yagi
Hiromi Shizu
Takashi Sano
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Toshiba Corp
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Toshiba Corp
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Priority to PCT/JP1993/000929 priority Critical patent/WO1995002313A1/en
Assigned to KABUSHIKI KAISHA TOSHIBA reassignment KABUSHIKI KAISHA TOSHIBA ASSIGNMENT OF ASSIGNORS INTEREST (SEE DOCUMENT FOR DETAILS). Assignors: ENDO, HIROSHI, FUJIMORI, YOSHINORI, HISANO, KATSUMI, IWASAKI, HIDEO, MOMMA, JUN, SAKAMOTO, TOSHIYA, SANO, TAKASHI, SASAKI, TOMIYA, SHIMOTORI, KAZUMI, SHIZU, HIROMI, SORI, NAOYUKI, YAGI, NORIAKI
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Abstract

A thermal conductivity sheet is provided which is superior all in heat radiating characteristics (thermal conductivity) in the direction of sheet thickness, close-contact with respect to parts to be cooled, and electrical insulation. In a thermal conductivity sheet 1 in which a plurality of highly thermally conductive insulators 3 are dispersed in a matrix insulator 2, the highly thermally conductive insulators 3 are oriented obliquely or erectly in the direction of thickness of the thermal conductivity sheet 1 such that at least one end faces of the highly thermally conductive insulators 3 are exposed to a surface of the matrix insulator 2. Preferably, a ratio of the total sectional area of the highly thermally conductive insulators 3 to the total surface area of the thermal conductivity sheet 1 is set to be equal to or larger than 1%.

Description

TECHNICAL FIELD
The present invention relates to a thermal conductivity sheet (heat radiating sheet, or heat releasing sheet), and more particularly to a thermal conductivity sheet which is superior in a heat radiation, electrical insulation and pliability or flexibility, which has excellent close-contact property with respect to electronic equipment parts such as transistors, capacitors and LSI packages, and which can efficiently dissipate or transmit the heat produced by parts to the exterior.
BACKGROUND ART
Electric/electronic parts such as transistors, capacitors and LSI packages tend to shorten the service life and deteriorate the reliability with the heat generated during operation. As measures for preventing Such drawbacks, it has been proposed to interpose a thermal conductivity sheet, which is superior in thermal conductivity and close-contact, between electric/electronic parts and a heat sink (cooling means), such as a heat radiating fin, thermally connected to the electric/electronic parts for dissipating the generated heat to the exterior through the thermal conductivity sheet.
The thermal conductivity sheet is generally manufactured by dispersing a thermal conductivity filler in a matrix resin and shaping the mixture into the form of a sheet. Silicone rubber, for example, is employed as the matrix resin, while boron nitride in the form of particles, plates and needles, for example, is employed as the thermal conductivity filler.
More specifically, the thermal conductivity sheet is manufactured using the above-exemplified materials of a thermal conductivity filler and a matrix resin by any of three primary methods below.
In the first method, a matrix resin (e.g., silicone rubber) and a thermal conductivity filler (e.g., boron nitride (BN)) are combined and mixed with each other to prepare a material mixture. The material mixture is then shaped into the form of a sheet by using rolls, a calender, an extruder or the like as with usual rubber materials. The shaped sheet is pressed and vulcanized.
In the second method, a matrix resin (e.g., silicone rubber) and a thermal conductivity filler (e.g., boron nitride) are mixed and diluted in a solvent. A resulting mixture is then formed into a sheet by a doctor
The sheet is dried, pressed and then blade process. vulcanized.
In the third method, a matrix resin (e.g., silicone rubber) of 100 weight parts and a thermal conductivity filler (e.g., boron nitride) of 200 or more weight parts are combined together to prepare a compound material containing the thermal conductivity filler at a high ratio. The material is mixed by using a closed type kneading machine such as a kneader to form a powdery rubber material. A predetermined amount of the powdery rubber material is filled in a mold for shaping into a sheet, following which the molded sheet is pressed and vulcanized.
FIG. 41 is a sectional view showing a structure of the conventional thermal conductivity sheet fabricated by any of the prior art manufacture methods described above. In a prior art thermal conductivity sheet 10, thermal conductivity fillers 12 are combined and distributed in a matrix resin 11 in a condition where the long axes of the thermal conductivity fillers 12 are oriented in the direction of plane of the thermal conductivity sheet 10 (the longitudinal direction thereof).
The thermal conductivity fillers 12 are oriented so longitudinally of the sheet because the fillers 12 are aligned in the direction of rolling or extrusion when the material mixture is rolled or extruded for shaping into the sheet.
The inventors have found that the thermal conductivity sheet fabricated by any of the prior art manufacture methods has a problem below. Since the thermal conductivity fillers 12 are oriented in the direction of sheet plane, there is a tendency that the adjacent thermal conductivity fillers 12 are contacted with each other and the thermal conductivity fillers 12 as a whole are substantially continuously extended in the direction of plane of the thermal conductivity sheet 10 (the longitudinal direction thereof). Accordingly, heat is easy to transmit in the direction of plane of the thermal conductivity sheet 10, but is hard to transmit in the direction of thickness of the thermal conductivity sheet 10. As a result, the thermal conductivity sheet has poor performance when it is used for the purpose of mainly utilizing its heat radiating characteristics in the direction of sheet thickness.
The inventors have also found the fact as follows. In a thermal conductivity sheet in which a large amount of brittle materials such as ceramics and metals are distributed as thermal conductivity filler in a soft matrix resin, the modulus of elasticity is increased and pliability is decreased. Therefore, when the thermal conductivity sheet is fitted to a part to be cooled, the thermal conductivity sheet 10 is difficult to deform following surface irregularities of the cooled part. This results in the problem that the thermal conductivity sheet 10 does not sufficiently closely contact with the surface of the cooled part, the thermal conductivity resistance is increased, and hence the heat radiating characteristics are deteriorated.
A thermal conductivity sheet disclosed in Japanese patent laid-open No. 54-163398 is formed of a composite of resin and boron nitride powder. The particle size of the boron nitride powder is set to be 0.2 to 1 time the sheet thickness, and the powder is filled into the sheet by pressing while it is also exposed to the sheet surface. Also, a thermal conductivity sheet disclosed in Japanese patent laid-open No. 3-20068 is of a structure that inorganic filler particles are arranged to lie continuously while contacting with each other. Any of the above-mentioned thermal conductivity sheets is intended to enhance thermal conductivity by increasing the density of fillers, which possess thermal conductivity, to such an extent that the fillers are present in the continuous form.
As a result of studies made by the inventors, however, it has been found that the thermal conductivity sheet lowers flexibility with an increase in a contact rate of the filler particles, and it eventually exhibits thermal conductivity as low as 4 W/m·K and slight flexibility and hence has a difficulty in drastically improving the thermal conductivity.
On the other hand, as disclosed in Japanese patent laid open No. 3-151658, there is known a thermal conductivity sheet in which small thermal conductivity fillers in the form of particles, plates, needles, etc. are oriented in the direction of sheet thickness such that the thermal conductivity fillers are contacted with each other, i.e., that the thermal conductivity fillers are arranged continuously with no resin layers therebetween.
In the above thermal conductivity sheet, however, since the thermal conductivity fillers are arranged continuously with no resin layers therebetween, flexibility (pliability) of the thermal conductivity sheet is impaired. Stated otherwise, in an attempt of increasing the density of the fillers to increase the thermal conductivity, it is inevitable for the thermal conductivity sheet becomes hard and brittle. Thus, the inventors have found that when the above thermal conductivity sheet is fitted to electronic/electric parts and heat sinks, the contact area is reduced, the contact thermal resistance is generated, and a sufficient degree of thermal conductivity cannot be obtained.
As another example in which thermal conductivity fillers are oriented in the direction of sheet thickness, a thermal conductivity member is disclosed in Japanese patent laid-open No. 62-240538. This thermal conductivity member is of a structure that metal short fibers or metal powders are planted or buried in an adhesive layer on a base sheet so as to form continuous heat radiating paths. When the metal short fibers are arranged in the direction of sheet thickness and the base sheet is made of an electrically conductive material, the electrically conductive material must be completely isolated by the adhesive layer in order that the thermal conductivity member has electrical insulation in its entirety. In this case, the thermal conductivity is improved by filling and dispersing metal powders in the adhesive layer.
As a result of studies made by the inventors, however, the following problems have been found. Because the amount of filled metal powders is restricted as a necessity to keep electrical insulation, the thermal conductivity is inevitably lowered. Also, the sufficiently large density of the filled metal powders entails a difficulty in ensuring flexibility. When the base sheet is made of the insulating materials disclosed in the above laid-open publication, the thermal conductivity of the thermal conductivity member as a whole is lowered and the effect of radiating and dissipating heat becomes insufficient. Further, when the thermal conductivity member, in which metal short fibers are planted in the adhesive layer on the base sheet, is fitted over an electric circuit, an electric trouble is apt to occur in that the metal short fibers may contact with each other to cause a short-circuit between a voltage applied portion and a ground potential portion on the circuit.
Additionally, as disclosed in Japanese patent laid-open No. 56-35494, there is known a thermal conductivity body in which a coating film comprising metal oxide particles dispersed in adhesive organic high molecules is formed on a highly thermally conductive base sheet. In this prior art, while the highly thermally conductive base sheet itself has sufficiently large thermal conductivity, the coating film consisted of a resin and harmless metal oxide particles except BeO dispersed in the resin has low thermal conductivity. Thus, it has been found from studies made by the inventors that the thermal conductivity body has low thermal conductivity as a whole and an effective heat radiating and dissipating action cannot be excepted.
Though the technical field is different from the present invention, various electrically conductive sheets having anisotropy are disclosed in Japanese patent laid-open No. 62-31909, No. 55-111014, No. 63-86322, No. 2-68811, No. 2-68812, for example. In any of the disclosed electrically conductive sheets, the sheet is given with anisotropy, while focusing on electric conductivity, by a structure that a conductive member is penetrated through the sheet in the direction of thickness thereof and the conductive member is exposed to the sheet surface. Stated otherwise, the disclosed electrically conductive sheets are intended to keep electric conductivity with high reliability, and are basically different in technical nature from the thermal conductivity sheet of the present invention which is intended to satisfy electrical insulation, thermal conductivity and flexibility at the same time.
Japanese patent laid-open No. 64-76608 discloses an electrically conductive member in which bumps are formed on an electrically conductive base material, and Japanese patent laid-open No. 1-286206 discloses an electrically conductive member in which a layer of a metal having the low melting point is formed on an electrically conductive portion. However, any of these prior parts has an object to ensure electrical connection of the electrically conductive member, and hence is different from the thermal conductivity sheet of the present invention aiming at thermal connection.
Furthermore, in the thermal conductivity sheet which is formed by arranging thermal conductivity fillers having the particle size of several μm to 10 μm such that the fillers lie continuously while contacting with each other with no resin layers therebetween, as disclosed in the above-cited Japanese patent laid-open publication (No. 3-20068), flexibility (pliability) of the thermal conductivity sheet is impaired. Therefore, the contact area of the thermal conductivity sheet with respect to electronic/electric parts and heat sinks (cooling means) is reduced and the contact thermal resistance is increased. Consequently, a sufficient degree of thermal conductivity cannot be expected.
In this way, as the dispersion density of thermal conductivity fillers is increased, the thermal conductivity is improved, but the flexibility of the thermal conductivity sheet is lowered. Then, it is difficult to obtain a thermal conductivity sheet which satisfies both high thermal conductivity and good flexibility, by any compounding techniques described above as the prior art methods.
With remarkable development of electronic/electric parts in recent years, an increase in integration, speed and output of electronic equipment, including semiconductor devices, is progressed and, correspondingly, the amount of heat generated from heat generating parts such as semiconductor devices is also increased. There is thus a demand for a thermal conductivity sheet which is more superior in heat radiating characteristics.
The present invention has been accomplished with a view of solving the problems described above, and its object is to provide a thermal conductivity sheet which is markedly superior in heat radiating characteristics (thermal conductivity) in the direction of sheet thickness, electrical insulation, and close-contact with respect to parts to be cooled.
DISCLOSURE OF THE INVENTION
To achieve the above object, the inventors have conducted experiments of distributing various highly thermally conductive insulators in matrix insulators, and confirmed influences of the orientation and the amount of filled highly thermally conductive insulators upon heat radiating characteristics. As a result, a thermal conductivity sheet having superior heat radiating characteristics has been obtained particularly when the highly thermally conductive insulators are arranged in the direction of thickness of the thermal conductivity sheet and at least one end faces of the highly thermally conductive insulators are exposed to a surface of the matrix insulator. The present invention has been accomplished based on the above finding.
More specifically, the present invention resides in a thermal conductivity sheet in which a plurality of highly thermally conductive insulators are continuously interconnected through a flexible matrix insulator, wherein the highly thermally conductive insulators are arranged obliquely or erectly in the direction of thickness of the thermal conductivity sheet such that at least one end faces of the highly thermally conductive insulators are exposed to a surface of the matrix insulator. Preferably, the highly thermally conductive insulators are arranged obliquely or erectly in the direction of thickness of the thermal conductivity sheet such that both end faces of the highly thermally conductive insulators are exposed to surfaces of the matrix insulator.
Here, the matrix insulator is made of, for example, a thermoplastic resin such as silicone rubber, polyolefinic elastomer, polyethylene, polypropylene, polystyrene, poly-p-xylene, polyvinyl acetate, polyacrylate, polymethacrylate, polyvinyl chloride, polyvinylidene chloride, fluorine-base plastic, polyvinyl ether, polyvinyl ketone, polyether, polycarbonate, thermoplastic polyester, polyamide, diene-base plastic, polyurethane-base plastic, silicone and inorganic plastic, or a thermosetting resin such as a phenol resin, furan resin, xylene/formaldehyde resin, ketone/formaldehyde resin, urea resin and epoxy resin.
On the other hand, the highly thermally conductive insulator is made of a material which has high thermal conductivity and also has electrical insulation, such as aluminum nitride, boron nitride, silicon nitride, silicon carbide, BeO, C--BN, diamond, HP--TiC and alumina ceramic. In addition to the above-mentioned insulator in the form of a single layer, the highly thermally conductive insulator may be formed by integrally laminating an insulating thin film (insulating layer) on a surface of a conductor. Here, the conductor means general kinds of metals and is made of at least one selected from among typical metals such as gold, silver, copper and aluminum. The insulating thin film may be made of a heat-resistant high molecular material or the like in addition to any of the above-mentioned materials for the matrix insulator.
The thickness of the insulating thin film is properly selected depending on the voltage applied to the thermal conductivity sheet as a final product, but is desirably equal to or less than 0.1 mm to prevent a deterioration in thermal conductivity characteristics of the conductor itself. In order to maintain electrical insulation, it is required for the insulating thin film to have electrical insulation resistivity (volume resistivity) equal to or larger than 1012 Ω-cm. The insulating thin film may be made of any of insulating ceramics such as aluminum nitride (AlN), boron nitride (BN), silicon nitride (Si3 N4), alumina (Al2 O3) and zirconium oxide (ZrO2).
The thermal conductivity of the highly thermally conductive insulators is set to be equal to or larger than 25 W/m·K.
It is to be understood that the long axes of the numerous highly thermally conductive insulators oriented obliquely or erectly in the direction of thickness of the thermal conductivity sheet are not all necessarily aligned in a certain direction, and the sheet may be of an oriented structure that the highly thermally conductive insulators oriented at various angles are mixed together.
With the thermal conductivity sheet thus constructed, since the highly thermally conductive insulators are arranged obliquely or erectly in the direction of thickness of the thermal conductivity sheet such that at least one end faces of the highly thermally conductive insulators are exposed to the surface of the matrix insulator, heat radiating paths with high thermal conductivity are formed continuously in the direction of thickness of the thermal conductivity sheet. It is therefore possible to effectively transmit heat in the direction of thickness of the thermal conductivity sheet, and to greatly improve the efficiency of cooling electronic/electric parts to which the thermal conductivity sheet is fitted.
Particularly, by arranging the highly thermally conductive insulators such that the insulators are oriented obliquely with respect to the direction of thickness of the thermal conductivity sheet, the sheet can have higher pliability and flexibility in the direction of sheet thickness than the sheet in which the insulators are oriented erectly. It is therefore possible to develop an effect of releasing stresses imposed from parts to be cooled, and to improve close-contact of the thermal conductivity sheet with respect to the cooled parts.
Also, by setting a ratio of the total sectional area of the highly thermally conductive insulators to the total surface area of the thermal conductivity sheet to be equal to or larger than 1%, the thermal conductivity sheet as a whole can have higher thermal conductivity than a thermal conductivity sheet which is made of only a general resin material.
It is to be understood that the highly thermally conductive insulators arranged in the matrix insulator are not all necessarily penetrated through the thermal conductivity sheet in the direction of sheet thickness, and the sheet may be of an arrangement that the highly thermally conductive insulators penetrating through the sheet and the highly thermally conductive insulators not penetrating through the sheet but lying horizontally or obliquely are mixed together.
To achieve a high predetermined value of thermally conductivity, however, it is required to adjust the total sectional area of the highly thermally conductive insulators penetrating through the sheet to be equal to or larger than 1% with respect to the total surface area of the thermal conductivity sheet. If the area ratio is less than 1%, the effect of improving the thermal conductivity of the thermal conductivity sheet would be poor.
Conversely, if the area ratio exceeds 90%, the thermal conductivity would be further increased, but the flexibility (pliability) of the thermal conductivity sheet would be lowered, the close-contact with respect to the cooled parts would be impaired, and the production cost would be raised because of the increased amount of expensive highly thermally conductive insulators used. For that reason, the area ratio is preferably set to fall in the range of about 1 to 90%, more preferably in the range of about 10 to 60%.
Particularly, when an AlN sintered body which has thermal conductivity of 200 W/m·K or more and a diameter of 0.5 mm is used as the highly thermally conductive insulator, the present thermal conductivity sheet having the area ratio set to be equal to or larger than 15% exhibits thermal conductivity two or more times that of the prior art ones. Despite of such a drastic increase in the thermal conductivity, the modulus of elasticity (Young's modulus) of the matrix insulator remains constant and the flexibility of the thermal conductivity sheet is not impaired. In this connection, it is important to distribute a large number of highly thermally conductive insulators each having a small sectional area uniformly all over the matrix insulator, rather than arranging a small number of highly thermally conductive insulators each having a large sectional area. Using highly thermally conductive insulators each of which is thin and has a small sectional area is also effective to further increase the pliability of the thermal conductivity sheet.
The highly thermally conductive insulators and the matrix insulator constituting the thermal conductivity sheet may be arranged with certain regularity so as to form the thermal conductivity sheet which has anisotropy in thermal conductivity and/or modulus of elasticity in the direction of plane of the thermal conductivity sheet. For example, a thermal conductivity sheet may be formed by preparing highly thermally conductive insulators and matrix insulators all of which has a length equal to the sheet width and arraying both the insulators alternately into a unitary structure. In this thermal conductivity sheet, continuous heat radiating paths are formed not only in the direction of sheet thickness but also in the axial direction of the highly thermally conductive insulators, enabling heat to be transmitted effectively. Further, this thermal conductivity sheet can be easily bent in a direction perpendicular to the axial direction and hence can be fitted to surfaces of cylindrical parts to be cooled form with a high degree of close-contact. When the thermal conductivity sheet is used in a condition of undergoing stress from a part to be cooled, the effect of releasing the stress is developed by fitting the thermal conductivity sheet to the part such that the acting direction of the stress agrees with the direction in which the thermal conductivity sheet has the low modulus of elasticity. As a result, the close-contact between the cooled part and the thermal conductivity sheet is maintained in a good condition for a long term.
For the thermal conductivity sheet using columnar highly thermally conductive insulators of which sectional areas are constant in the axial direction thereof, there is a risk that, for example, when the thermal conductivity sheet is deformed upon external forces applied to the sheet, the highly thermally conductive insulators are apt to easily slip off from the matrix insulator and the heat radiating capability may be lowered. Therefore, by forming the highly thermally conductive insulators into, e.g., a barrel or a hyperboloidal drum such that sectional areas of the highly thermally conductive insulators are changed in the axial direction thereof, the insulators can be effectively prevented from slipping off from the matrix insulator.
Furthermore, the highly thermally conductive insulator comprises a plurality of columnar highly thermally conductive insulator elements which are adjacent to each other in the direction of thickness of the thermal conductivity sheet with their central axes offset from each other, and a coupling element for integrally coupling the adjacent columnar highly thermally conductive insulator elements in the direction of plane of the thermal conductivity sheet.
In the thermal conductivity sheet of the above structure, when the sheet is subject to pressing force in the direction of sheet thickness, the highly thermally conductive insulator elements can be easily deformed at the coupling elements to flex to some extent. Therefore, the thermal conductivity sheet can easily release stresses and can also easily deform as a whole in conformity with surface configurations of parts to be cooled, thereby improving a degree of the close-contact. Particularly, by forming the coupling element to have a thickness equal to or smaller than 1/2 of the height of the columnar highly thermally conductive insulator elements, the thermal conductivity sheet is allowed to more easily flex at the coupling elements, which increases the elasticity and pliability of the thermal conductivity sheet as a whole and hence provides the further improved close-contact with respect to the cooled parts.
With another structure that the highly thermally conductive insulator comprises a plurality of highly thermally conductive insulator elements which are interconnected in the direction of thickness of the thermal conductivity sheet, and the highly thermally conductive insulator elements adjacent to each other are freely movable relatively at contact surfaces therebetween, when the thermal conductivity sheet is subject to external stresses, the individual insulating elements are independently movable at the contact surfaces in both the directions of thickness and plane of the sheet, and can provide the thermal conductivity sheet with higher pliability than the sheet employing the highly thermally conductive insulators each of which is in the form of a single column. Accordingly, even if a cooled part such as an electronic/electric part has irregularities on its surface, the thermal conductivity sheet can satisfactorily closely contact with the part surface and can posses superior heat radiating characteristics for a long term.
In this connection, the contact surfaces of the insulator elements adjacent to each other may be not only parallel to the direction of plane of the thermal conductivity sheet, but also inclined with respect to the direction of sheet plane or formed to be saw-toothed in section. By so forming the contact surfaces, when adjacent insulator elements are relatively displaced by external forces, the adjacent filler elements are kept partly contacted with each other and a possibility of losing the heat radiating paths is small.
Also, the height between both end faces of the highly thermally conductive insulators may be set to be smaller than the thickness of the matrix insulator so that recessed steps are formed between surfaces of the matrix insulator and the end faces of the highly thermally conductive insulators.
With the thermal conductivity sheet having such recessed steps formed thereon, the surfaces of the soft matrix insulator are somewhat higher than both the end faces of the highly thermally conductive insulators exposed to the surfaces of the matrix insulator. Therefore, even when the thermal conductivity sheet is fitted to a cooled part having irregularities on its surface, the projecting soft matrix insulator is allowed to deform following the irregularities on the cooled part surface so that the end faces of the highly thermally conductive insulators and the surface of the matrix insulator can be both closely contact with the cooled part surface to effectively transmit the heat.
In addition, convex bumps made of a soft metal may be formed on the end faces of the highly thermally conductive insulators exposed to the surfaces of the matrix insulator. Used as the soft metal is, e.g., a metal having the low melting point equal to or lower than 200° C. such as Bi--Pb, Bi--Pb--Sn, Bi--Sn--Cd, Bi--Sn--Zn, Bi--Cd, and Pb--Sn.
With the thermal conductivity sheet having such bumps formed thereon, when the thermal conductivity sheet is press-fitted to cooled parts, the bumps are collapsed so as to deform following irregularities on surfaces of semiconductor devices and heat radiating fins. Therefore, a degree of close-contact between both the members is increased and the contact thermal resistance can be greatly reduced.
The thermal conductivity sheet according to the present invention is manufactured by a manufacturing method for a thermal conductivity sheet in which highly thermally conductive insulators are arranged in a matrix insulator, the method comprising the steps of arranging highly thermally conductive insulators in the predetermined form obliquely or erectly in the direction of thickness of the thermal conductivity sheet, covering both end faces of the highly thermally conductive insulators with a masking agent, coating the matrix insulator around the highly thermally conductive insulators except the covered both end faces to prepare a preform, removing the masking agent from the preform, and heating and shaping the preform into a sheet. Here, a paraffin, styrene rubber or the like is used as the masking agent.
Alternately, in a manufacture method for a thermal conductivity sheet in which highly thermally conductive insulators are distributed in a matrix insulator, the method may comprise the steps of arranging a number of highly thermally conductive insulators with spacings therebetween such that their long axes are parallel to each other, coating the matrix insulator around the highly thermally conductive insulators to prepare a block-like preform, and slicing or cutting the block-like preform at a predetermined cut angle with respect to the long axes of the highly thermally conductive insulators to prepare a plurality of thermal conductivity sheets.
With the above manufacture method, by cutting (slicing) the block-like preform at a cut angle of 90 degrees with respect to the long axes of the highly thermally conductive insulators, the thermal conductivity sheet in which the highly thermally conductive insulators are erected in the direction of sheet thickness can be mass-produced.
On the other hand, by setting the cut angle to an acute angle, the thermal conductivity sheet in which the highly thermally conductive insulators are inclined with respect to the direction of sheet thickness and which has superior elasticity can be mass-produced. If the cut angle is less than 30 degrees, the heat radiating paths defined by the highly thermally conductive insulators would be long and the cooling capability of the thermal conductivity sheets would be poor. Therefore, the cut angle is preferably set to be in the range of 30 to 90 degrees. Particularly, to obtain the thermal conductivity sheet in which the highly thermally conductive insulators are inclined with respect to the direction of sheet thickness and which has superior elasticity, the cut angle is set to be in the range of 30 to 60 degrees.
In any of the above manufacture methods, by applying a coating agent, which contains a lipophilic group, over surfaces of the highly thermally conductive insulators to form coating layers, prior to the step of coating the matrix insulator around the highly thermally conductive insulators, wetting between the matrix insulator and the highly thermally conductive insulators is improved and the content of the highly thermally conductive insulators in the entire thermal conductivity sheet can be increased without reducing the bonding strength therebetween. It is hence possible to optionally prepare the thermal conductivity sheet which has any desired thermal conductivity.
In the thermal conductivity sheet according to the present invention, as described above, the highly thermally conductive insulators are not simply filled and compounded in the matrix insulator, but the thermal conductivity sheet comprises portions possessing thermal conductivity and portions possessing flexibility which are distinctly separated in terms of functions, while ensuring electrical insulation of the thermal conductivity sheet as a whole. With such a feature, the thermal conductivity, the flexibility and the electrical insulation can be appropriately adjusted in design. Consequently, the thermal conductivity sheet which has all of the thermal conductivity, the flexibility and the electrical insulation can be provided.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a sectional view showing a first embodiment of a thermal conductivity sheet according to the present invention.
FIG. 2 is a perspective view of the thermal conductivity sheet shown in FIG. 1.
FIG. 3 is a perspective view showing one exemplified configuration of highly thermally conductive insulators for use with the thermal conductivity sheet shown in FIG. 1.
FIG. 4 is a graph showing the relationship between a ratio of the total sectional area of the highly thermally conductive insulators to the surface area of the thermal conductivity sheet and a ratio of the thermal conductivity of the thermal conductivity sheet to the thermal conductivity of the matrix insulator.
FIG. 5 is a sectional view showing a second embodiment of the thermal conductivity sheet according to the present invention.
FIG. 6 is a sectional view showing a third embodiment of a thermal conductivity sheet according to the present invention.
FIG. 7 is a perspective view showing one embodiment of a manufacturing method for the thermal conductivity sheet according to the present invention.
FIG. 8 is a sectional view showing a fourth embod